1. Field of the Invention
The present invention pertains generally to the field of optical lenses and more specifically, to a line lens or beam spreader lens for converting a narrow, round beam of radiation into a uniformly dispersed sheet of radiation.
2. Prior Art
A lens which converts a narrow, round beam of radiation into a uniformly distributed sheet of radiation finds its principal usefulness in military applications such as laser scan radar. It is well-known that a round beam of radiation when passed through a cylindrically curved refracting surface will be dispersed into a sheet of radiation. However, a number of critical deficiencies are encountered. For example, the span or width of the sheet obtained from practical curvatures of the refracting surface is inadequate in that it does not satisfy the normal requirement for a 90 degree span. Furthermore, if the curvature of the cylinder is sufficiently high, portions of the beam incident on the high angle regions of the surface are severely attenuated and in fact, radiation gaps are produced because the critical angle for total internal reflection will be exceeded for certain portions of the surface. As a result, a relatively non-uniform radiation pattern is produced which may in fact, comprise a plurality of non-radiation regions interspersed therethrough. Still an additional problem encountered in attempting to use a cylindrically curved refracting surface to generate a sheet of radiation from an incident beam, results from the awkward mechanical problems associated with the attempt to concentrate the incident beam over the span of the lens.
A prior art beam spreader lens has overcome the aforementioned problems of a cylindrically curved refracting surface. More specifically, a prior art lens having lenticular elements that are alternately concave and convex has been developed out of an extremely high index of refraction material (LASF9 having an index of refraction of 1.85). The extremely high index of refraction appears to solve the first two of the aforementioned disadvantages of the cylindrically curved refracting surface in that the span or width of the radiation sheet obtained from this prior art lens appears to be substantially adequate and relatively uniform without any severely attenuated segments. Furthermore, the lenticular configuration of the lens appears to overcome the aforementioned centration problem wherein a centrally directed incident beam is all that is required. Unfortunately, the prior art lens introduces a significant new disadvantage particularly in certain military applications where a lens of this type finds its most advantageous application. More specifically, the exposure of the aforementioned prior art lens to any form of heating, darkens the material and changes its optical characteristics rendering its transmission capabilities inadequate for the purpose to which it is applied. For example, in some applications the lens is exposed to atmosphere in a high velocity flight vehicle and in such applications, the air resistance is sufficient to heat the lens to an extent that the aforementioned detrimental optical characteristic changes occur. Consequently, the aforementioned high index of refraction material is not available for use as a line lens wherever that lens may be heated in its application.
There is however a far more suitable material available, namely, glass ceramic sold under such trademarks as ZERODUR and CERVIT. Such glass ceramic is resistant to heating in that it does not substantially change its transmission or other optical characteristics as a result of significantly higher temperatures. Unfortunately, ZERODUR for example has an index of refraction of only 1.544. As a result, the lens structure and geometry utilized to produce the prior art line lens using the aforementioned high index of refraction material, would not produce the same uniformly distributed 90 degree sheet beam in a line lens manufactured from glass ceramic. Consequently, the problem to be solved by the present invention is to provide a line lens which takes advantage of the aforementioned prior art line lens to the extent that it continues to satisfy the requirements of uniform sheet radiation distribution over a 90 degree field with a structural configuration that may be readily produced and at the same time permit the use of glass ceramic which will resist the aforementioned heating effect induced problems despite the significantly lower index of refraction compared to the prior art material discussed above.